Recently, as a consequence of a demand for standardization of digital communication networks, many of those networks are synchronous networks, and reliability of a synchronizing signal responsible for synchronization is called for. Known synchronous communication networks include the one using an optical fiber cable for performing transmissions of high-speed digital signals. In such a synchronous communication network, an oscillator for generating a main clock is provided in a system. This main clock is shared by both the transmitting side and the receiving side. Normally, a plurality of input signals, by being subjected to a plurality of times of hierarchical multiplexing processes, are converted into a high-speed multiplexed signals before transmission. The input signals are multiplexed in a byte unit. Upon each multiplexing, signal transmission rate increases.
One of the known high-speed transmission network utilizing byte-multiplexing is SONET (synchronous optical network). As shown in FIG. 1, an STS-1 signal of this SONET system is configured such that one frame is constructed of 6480 bits (=90 bytes.times. 8 rows.times.8 bits), where 1 byte represents 8 bits. The duration of one frame is 125 .mu.s, and the bit rate is 51.84 MHz. The frame format of the STS-1 signal shown in FIG. 1 is provided in each channel. The headmost 2 bytes of the frame format are frame synchronizing patterns A1, A2, and the next 1 byte is a channel identification pattern C1. SOH (section overhead), LOH (line overhead), and POH (path overhead) are control data added to the information to be transmitted.
A plurality of STS-1 signals having the above frame format are subjected to a simple byte-multiplexing (meaning that no format conversion is carried out). FIG. 2 shows how three STS-1 signals are byte-multiplexed. The STS-1 signals for three S channels #1, #2 and #3 are byte-multiplexed so that an STS-3 signal having 155.52 MHz rate is generated. This STS-3 signal is standardized as an STM-1 signal according to the CCITT recommendation. It is assumed that an STS-1 signal is transmitted as an optical signal. At the head of the data in the three channels #1-#3, the 2-byte frame synchronizing patterns A1, A2 and the 1-byte channel identification pattern C1 are added. As indicated by broken lines, the STS-3 signal is formed by byte-multiplexing. It is to be noted that no frame patterns unique to the resultant STS-3 signal are inserted. Byte-multiplexing in this system is carried out such that the heads of the signals on the channels #1-#3 are in sync with each other, with the result that the frame-multiplexed synchronizing pattern of an STS-3 signal is of a 6-bit construction.
The frame synchronizing patterns A1, A2 are the same for the channels #1-#3, A1 being "11110110" and A2 being "00101000". The channel identification patterns C1 for the channels #1-#3 are set to be different from channel to channel.
Referring back to FIG. 1, B1-B3 are byte interleave parities; C2 is a signal label byte indicative of the presence or absence of information; D1-D12 are data communication bytes for transporting, for example, information relating to the state of different units; E1, E2 are order wire bytes; F1, F2 are user channel bytes; G1 is a path status byte for detecting a parity error of an input signal and returning the detected error to the originating unit; H1, H2 are pointers having a stuffing function for incorporating an asynchronous system; H3 is a pointer having a variable slot function in stuffing; H4 is a multi-frame indication byte; J1 is a trace byte; K1, K2 are automatic protection switch bytes; and Z1-Z5 are reserved bytes.
At the receiving side, frame synchronization is effected by detecting the 6-byte frame multiplexed synchronizing pattern of the STS-3 signal as shown in FIG. 2. As indicated by the broken lines, the signal is divided into the data for each of the channels #1-#3, whereupon the channel identification pattern C1 is utilized to determine whether the accurate separation is achieved.
It is possible to further multiplex the STS-1 signal. In a manner similar to the above, the frame synchronizing patterns A1, A2 and the channel identification pattern C1 at the head of the STS-1 signal are byte multiplexed and provided at the head of the STS-n signal obtained as a result of n-multiplexing. The frame multiplexed synchronizing patterns provided at the head of the STS-n signal is of a 2 n byte construction.
As has been described, the conventional synchronizing data communication network allows only one synchronizing signal source in the network and is merely capable of informing to the outside that a failure has occurred and the timing fails to be maintained. Accordingly, data communication is disabled when a failure occurs and prevents the timing to be maintained.